These are what is commonly referred to as the "hat lights".
These indicators are called "relampable" as the lens unscrews in order to replace the midget flange bulb.
They were intended for incandescent bulbs, however, modern LED bulbs are available as well.

Options for the lenses included 5 colors Blue, Amber, Green, White, and Red. Some lenses have been found with numbers stamped on the top. It seems as if there were more options in the past as to whether to have a more translucent or opaque lens. The translucent lenses pictured are no longer manufactured or available.

In GB1 the configuration is as follows
Hero Gun
1 Solid orange on gun tip
1 Solid orange by vent pushed out from the inside
1 Translucent white on gun box step

The GB2 configuration is as follows
Same as above for carry over converted GB1 to GB2 packs
*note* there is a GB1 pack that was recently sold as Harold Ramis' pack that was retrofitted for GB2 that had the translucent hat light mounted from the top by the vent although not seen in this state in GB1 movie.

GB2 Semi Hero Packs
1 Solid Orange on tip
1 Translucent white by vent mounted from the top. (this was done as the throwers were made of fiberglass)
1 Translucent white on gun box step

*note one semi hero has been seen with the translucent white cap with a number imprinted on it

GBTVG gun
1 Red on tip
1 Green by vent
1 White on gun box step

The socket comes in an insulated model or a standard model but both are practically the same other than they come with extra parts for the threading section. There was also an option to have the sockets use a color code on the bottom so it would be easier to identify from the back of a panel.

This style indicator is still produced but it requires a special order in large quantities.Current lenses and sockets are typically available in the GBFans shop.
These were originally used in aircraft so they surface from time to time in surplus/salvage yards.

The correct type of bulb is a Miniature bulb midget flange base T1-3/4